1. Field of the Invention
The present invention relates to a CO gas sensor for measuring a concentration of carbon monoxide (CO) gas contained in air or an exhaust gas from a combustion engine or the like and a CO gas concentration measuring apparatus using said CO gas sensor, and more particularly to a CO gas sensor that is operatable at a high temperature of 600 to 900.degree. C. with reducing an adverse affect of coexistence oxygen (O.sub.2) to a CO gas measurement value, and a CO gas concentration measuring apparatus using said CO gas sensor.
2. Related Art Statement
There are many poisoning and fatal accidents caused by CO gas generated by an incomplete combustion in an apparatus for burning fossil fuel, i.e., heater equipment such as a petroleum stove, a petroleum or oil fan heater, a gas heater or a hot water supplying equipment for heating water. This is because the human being can not notice the generation of the CO gas since CO gas has no color and no smell. Also, in a room or the like in which the above-described equipment is installed, since CO gas is a gas having substantially the same molecular weight as that of air, CO gas is diffused uniformly in the air. As a result, the human being unconsciously inspires CO gas, so that the inspired CO gas is combined directly with hemoglobin in the blood. In this case, he or she can not continue the inspiration of oxygen to cause a suffocation.
On the other hand, as described above, CO gas is a harmful gas to the human body but is useful in the industrial field. For example, methanol is synthesized from hydrogen (H.sub.2) gas and CO gas and used as material for various chemical products.
It is desired to reduce the concentration of CO gas discharged from an internal combustion engine or a combustion furnace for reasons of various regulations on the components in exhaust gas and a more effective use of fossil energy resources which are derived from recent environmental problems. For this reason, the composition of an exhaust gas is analyzed so as to optimize the combustion efficiency and the operation conditions by, for example, feeding back and processing the exhuast gas. Also, the on-site tests and researches of a power generating system using a fuel cell having a higher energy conversion efficiency than that of a conventional power generating system using a turbine have steadily progressed. In such a fuel cell, a mixture gas of CO gas and hydrogen (H.sub.2) gas produced by reacting water steam under the presence of catalyst or a commercial gas mainly composed of methane (CH.sub.4) is used as fuel.
As described above, a variety of combustion equipments requires installation of sensors for measuring a concentration and detecting the generation of CO gas. Also, the maintenance and investigation of these combustion equipments requires portable CO gas concentration measurement devices. Also, in a chemical plant or a combustion engine or the like, it is preferable to always monitor a CO gas concentration for controlling suitably the CO gas concentration.
Main kinds of such CO gas sensors are classified into a semiconductor type, a catalyst combustion type, a solid electrolyte type and a thermal transfer type. Among those, the solid electrolyte type CO gas sensor is based upon the measurement principle, i.e., the electromotive force measurement for a rich/lean cell, is composed of a metal electrode and a stabilized zirconia having an oxygen ion transfer property, and is superior in heat resistance, shock resistance and poisoning resistance.
However, since the CO gas sensor using the stabilized zirconia is also an oxygen sensor, it is difficult to selectively measure a CO gas concentration in a gas in which oxygen coexists. Also, in a sensor using a platinum electrode which is used in general, the oxidation of CO gas is rapid on the platinum electrode surface at such a high temperature of 600 to 900.degree. C.
Therefore, it is difficult to distinguish the measurement results attributed to oxygen and CO gases, in the case where oxygen is contained in a gas to be measured respectively.